1. Field of the Invention
The present invention relates generally to a backlight unit, and more particularly to an elastic frame for holding a light guide plate and the like, and a method for assembling the same. The back light unit can, for example, be used in a liquid crystal display device.
2. Description of the Related Art
A liquid crystal display device is widely used in various fields of electrical equipment as a flat-screen display device, providing advantages such as lightness, thin profile, and low power consumption. A transmissive type liquid crystal device includes a back light unit having a surface light source for illuminating a liquid crystal panel from a back side. Such transmissive type liquid crystal devices display pictures, in which a light from the back light unit is selectively transmitted through each cell of the liquid crystal device to the viewer.
The backlight unit used as a surface light source is classified into two types, an edge emission type, and a direct emission type, depending on where the light source is located. For example, the edge emission type includes a plurality of line light sources such as cold cathode fluorescent lamps (CCFLs) and a plurality of point light sources (LEDs) for emitting light to an edge of a light guide plate that faces the liquid crystal display panel. On the other hand, the direct emission type includes a plurality of line shaped light sources such as CCFLs arranged on a rear side of the liquid crystal display panel and a diffusion panel between the liquid crystal display panel and the light source. The edge emission type has an advantage of slimness and is suitable, for example, for displays implemented into a mobile electrical equipment or a note type personal computer.
In the back light unit of the edge emission type, the light guide plate and the light source are held by a frame for surrounding the light guide plate. Thus, a construction is proposed to sandwich the light guide plate between a frame and a back cover at a peripheral region. Another construction, to attach the light guide plate to the frame by a double stick tape, is also proposed.
FIG. 10 shows one example of a construction in which a light guide plate is sandwiched between a frame and a back cover. In this example, a light guide plate 103 and a reflector 104 are held between a rigid plastic frame 101 and a back cover 102 named as a rear vessel and an optical sheet 105 is arranged on the light guide plate 103.
FIG. 11 shows another example of a construction in which a light guide plate 202 is adhered to a frame by a double stick tape. In this example, the light guide plate 202 is arranged on a support element 201a provided at a rear portion of a frame 201 made of rigid plastic and extending inside a display region. The light guide plate 202 is adhered to the support element 201a by a double stick tape 203. A reflector sheet 204 is inserted between the light guide plate 202 and the support element 201a and an optical element 205 is arranged on the light guide plate 202.
Further, Japanese laid open patent application 2006-216244 discloses a construction for holding a light guide plate in which first and second internal frames both made of metal are paired with each other so as to hold the light guide plate. A first metal internal frame of a clip shape has three edges for holding the light guide plate, except one edge for receiving emitted light from a light source such as LEDs. On the other hand, the second internal frame made of metal that contains a light source is fitted along a light incident edge of the light guide plate so as to maintain a gap between the emitting surface of LEDs and a light incident surface of the light guide plate.
However, if the construction in which the light guide plate is held by combining the frame and the back cover is adopted, disadvantages such as high cost and weight gain result because the back cover is expensive and heavy. Further, time to assemble is high because the light guide plate is fitted to both of the frame and the back cover.
On the other hand, if the construction in which the light guide plate is attached to the frame by a double stick tape is adopted, a lack in uniformity of brightness may be observed because a light absorption condition of the light guide plate changes depending on the way the double stick tape is attached. Furthermore, if the tape is directly adhered to the light guide plate to hold the light guide plate by the frame, spots in an image due to non-uniformity of adhesion of the tape may be generated, and which also may result in a lower luminance because light is absorbed by the tape.
The construction shown in the Japanese Laid Open Patent Application 2006-216244 may result in high cost and weight gain because the first and second internal frames are made of metal. Further, it is required to fit the first and the second frames made of rigid metal to the light guide plate, respectively. Therefore, the assembly of the backlight unit is complicated and the completed backlight unit is unstable in use because the first and second internal frames are not formed integrally.